This invention relates to the manufacture of photovoltaic cells and more particularly to the manufacture of thick film silver contacts on silicon solar cells.
Photovoltaic cells (xe2x80x9csolar cellsxe2x80x9d) directly convert radiant energy from the sun into electrical energy. One mode of manufacturing such devices commences with provision of semiconductor substrates in the form of doped silicon sheets or wafers. By way of example, the solar cell substrates may be rectangular wafers cut out of EFG-grown polycrystalline silicon bodies. Typically such wafers are of p-type conductivity and have a thickness in the range of 0.011 to 0.015 inches. Other substrates may be used to make silicon solar cells, e.g., circular or square single crystal silicon substrates and rectangular cast polycrystalline silicon substrates.
The manufacture of solar cells using polycrystalline or single crystal substrates involves a number of steps, including formation of a shallow p-n junction and deposition of a thin electrically-insulating anti-reflection (xe2x80x9cARxe2x80x9d) coating on one surface of the wafer (the xe2x80x9cfrontxe2x80x9d surface of the wafer). Typically the p-n junction is formed by a diffusion process and is located about 0.3-0.5 microns below the xe2x80x9cfrontxe2x80x9d surface of the wafer, and the AR coating consists of a silicon nitride coating with a thickness in the range of about 600 to about 1000 (usually about 800) Angstroms that fully cover that front surface. It is to be noted that the silicon nitride AR coating may be replaced by an oxide of silicon or titanium that also is transparent to solar radiation. For the purposes of this invention description, substrates that have been processed to include a p-n junction and an AR coating are identified hereinafter as xe2x80x9csolar cell wafersxe2x80x9d.
The solar cell wafers are converted to finished solar cells by forming electrical contacts (sometimes referred to as xe2x80x9celectrodesxe2x80x9d) on their front and rear surfaces whereby to permit recovery of an electrical current from the cells when they are exposed to solar radiation. Various materials have been used as electrical contacts for solar cells, the most common being silver, aluminum and nickel.
The contact on the front surface of the cell is generally in the form of a grid, comprising an array of narrow elements known as xe2x80x9cfingersxe2x80x9d and at least one elongate bus (also called a xe2x80x9cbus barxe2x80x9d) that intersects the fingers. The width and number of the fingers and busses are selected so that the area of the front surface exposed to solar radiation is maximized. The front contact comprises silver as the electrically conductive constituent.
Both thin film and thick film techniques have been used for the formation of electrical contacts on solar cells. Thin film manufacturing techniques include vapor deposition, sputtering and electroless plating. Thin film techniques permit the formation of pure metal contacts, with the result that the contacts exhibit excellent electrical properties. Thick film technology involves using a somewhat viscous metal-containing paste (also referred to as a metal-containing ink) to form a relatively thick metal-containing film on the substrate, and then firing that film so as to make an electrically-conductive layer that is strongly bonded to the substrate.
A number of techniques exist for forming thick film electrically-conductive components on solar cells and other semiconductor substrates, including screen printing, pad printing, and direct writing. Pad printing thick film components is described in U.S. Pat. No. 5,151,386, issued Sep. 29, 1992 to Frank J. Bottari et al. for xe2x80x9cMethod Of Applying Metallized Contacts To A Solar Cellxe2x80x9d). Direct writing of thick film components is disclosed in U.S. Pat. No. 5,151,377, issued Sep. 29, 1992 to Jack I. Hanoka et al. for xe2x80x9cMethod For Forming Contactsxe2x80x9d. The disclosures of the foregoing patents are incorporated herein by reference. A preferred arrangement is to form the back contact by pad printing and to form the front contact by direct writing
The direct writing method disclosed and claimed in said U.S. Pat. No. 5,151,377 is a preferred technique since it can be integrated into a mass production manufacturing process and is capable of forming front grid-type contacts with tall as well as narrow fingers. Making the fingers narrow is desirable to minimize light shadowing (i.e., to expose as much as possible of the front surface of the solar cell to solar radiation). Increasing the height of the fingers without a corresponding increase in finger width is desirable so as to maximize the current-carrying capacity of the fingers. The writing involves use of an ink that comprises a mixture of silver particles and a glass frit in a vehicle that comprises a pyrolyzable organic binder and an organic solvent that evaporates at a relatively low temperature. A suitable ink is described in said U.S. Pat. No. 5,151,377. The ink is used to print the grid pattern on the silicon nitride coating, after which the solar cell wafer is fired to cause the ink to penetrate the silicon nitride and form a front electrical contact bonded to the silicon substrate.
The present invention concerns a problem encountered in producing front grid-type contacts by printing a thick film grid contact pattern on the silicon nitride AR according to the method disclosed in U.S. Pat. No. 5,151,377 (cited supra). The finger sections of the grid pattern are printed after the bus bar portions. Prior to being fired the tall and narrow fingers formed by the ink are fragile and susceptible to disruption. The problem consists of at least a portion of one or more fingers of the printed grid pattern peeling away from the silicon nitride surface as a consequence of sensitivity to moisture and also because of the surface condition of the silicon nitride coating. This peeling occurs while the ink is still wet and is evidenced by voids in the fingers portion of the grid contact after the wafer has exited the firing furnace. It appears that the presence of moisture tends to make the fingers narrower, and this contraction reduces the contact angle of the ink relative to the silicon nitride, which in turn results in a reduction of the adherence or bond strength of the ink to the silicon nitride. The peeling phenomenon results in loss of performance or outright rejection of a number of finished solar cells due to imperfect grid contacts.
Accordingly the primary object of the invention is to provide a treatment that substantially reduces or eliminates the tendency of narrow elements of a wet ink contact pattern of the type described to peel away from an underlying silicon nitride layer.
A more specific object is to adjust the condition of the surface of the silicon nitride AR layer of a solar cell wafer in a manner that effectively reduces the tendency of the narrow fingers of a printed ink grid electrode pattern to peel away before or during the subsequent firing required to convert the printed pattern into an effective electrical contact.
Another object is to achieve the foregoing objects by a method that is incorporated into a solar cell manufacturing process sequence without requiring any other modification of that sequence.
Still another object is to provide a method of manufacturing photovoltaic cells comprising printing and firing of electrical contacts on the front and back sides of solar cell wafers having a silicon nitride coating on their front sides, characterized by pre-treating the silicon nitride coating of each wafer with an ionized air stream produced by a corona discharge prior to printing on the wafer""s front side.
The foregoing objects are achieved by subjecting the surface of the silicon nitride layer to a corona discharge surface treatment that effectively increases the wettability of that surface by the printing ink. The corona discharge surface treatment is accomplished using an apparatus that produces a plasma jet and directs it at the silicon nitride surface. It is believed that corona discharge treatment increases the surface energy level of the silicon nitride layer, and that the increased surface energy results in the improved wettability. The improved wettability is evidenced by improved adherence of the thick film ink used to form the printed grid-type contact pattern, with the result that lifting or peeling of the fingers of the grid-type printed pattern prior to firing is substantially eliminated. The printed grid contact pattern remains intact throughout the subsequent high temperature firing to create a bonded high performance electrical contact. Other features and aspects of the invention are described or set forth in the following detailed description of the invention which is to be considered together with the accompanying drawings.